VOLUME 2, NOVEMBER 2009
Mohd Ikhwan Hadi Bin Yaakob
Postgraduate Student (PhD)
M.Sc. Physics, Instrumentations (UTM)
B.Sc. Physics, Industrial Physics (UTM)
Micromachined Parametric Array Acoustic Transducers for Sonar Applications
Generally, when a sensor or transducer become smaller, sensing area will also shrunk and the amount of sample substances required to produce electrical output will be minimal. The consequences might be good and might also be bad. The former is making sense when sensitivity and resolution of the transducer are taken into account. The latter on the other hand, start taking place when operating environment is noisy. In terms of dimension, the advantage depends on the applications. Underwater platforms exist in various sizes and dimensions. Each platform carries specific number of task and functions.
Power capacity for every platform would be differ as well. For instance, utilizing the array of kilowatts tonpilz acoustics transducers with several meters in diameter inside the Akula class submarine has never been an issue, since the capacity of the platform meets performance requirements of the sensing unit. In contrast, compact platforms such as torpedo, ROV, AUV, underwater glider and surface vessel need compact and robust acoustic sensing mechanism in order to meet desired sounding resolution with limited spaces and power capability. In terms of intensity or SPL, it is odd to compare between conventional transducer designs and MEMS based transducers. However as stated earlier, comparison of sensitivity, directivity and resolution might be even.
Advantages of MEMS
In general, advantages of MEMS compare to macro devices and systems (not all of them) come in many different ways other than sizes. By adopting the same fabrication technologies with microelectronics and IC, total development cost of micro scale devices are significantly reduced. Cheap silicon based materials also contribute to the effectiveness of the development and fabrication cost. Developed transducer is possibly match to be integrated with any support electronics within the same chip. The integration will at least reducing the parasitic capacitive noise level on the systems.
The advantages of this technology have become more apparent over several limitations that exist in conventional transducer design. Operating frequency of conventional acoustic transducer was found too depending on the thickness of the piezoactive materials. As discussed on two previous issues, both conventional hydrophone and projectors contain a layer of piezoactive layer in order to transmit or receive acoustic signals by the means of electrical. Dependable on material thickness had narrower the margin of design flexibility and thus limiting the operational bandwidth of the transducer.
Bandwidth of the transducer determines the range of usable frequencies of acoustics signal that can be detected and projected. For sonar applications, wider bandwidth might possibly place a single transducer over several applications. In underwater communications, broad range of operating frequencies will increase the data transfer rate thus allowing fast and clear underwater communication channel. Another issue on conventional design is impedance mismatch. Without a matching material, maximum power transfer from the transducer to the load (as well as from the load to the transducer in hydrophone) will be impossible. Two pre-requisites properties for the matching layer are acoustic impedance of the material and the thickness of the matching layer itself. The simplest calculation for matching layer thickness is quarter of the wavelength of the signals for maximum power transfer. For high frequency operations that employ shorter pulses of signal, quarter wavelength will be less than 1 mm thick. Using standard machining and cutting, it is impractical to fabricate such layer thickness. In case of complex array, wiring every element in the array is one of the most tedious jobs.
MEMS based acoustic sensor however, able to operate in relatively wider frequency band and resonance frequency tuning can be controlled easily without fully depending on the thickness of piezoactive material. Previous study  found that the resonance frequency of MEMS b
ased acoustic device with vibrating diaphragm and membrane can be controlled by varying the overall thickness of the diaphragm. A few years later , the width of the membrane was proved to contribute more in shifting not only the resonance frequency, but the acoustic impedance and the coupling coefficient of the transducer. These features give a MEMS based transducer more design flexibility tailing by various underwater applications. By adjusting the width of the diaphragm while keeping the thickness value constant, acoustic impedance of the device can be shifted to be near the acoustic impedance of the load. For underwater applications, approximate acoustic impedance of sea water is around 1.5 MRayl.
Matched impedance ensures maximum energy transfer and increase the total efficiency of the sensor system, made this type of transducer suitable for platforms with limited power resources. From the perspective of fabrications, MEMS technology allows two dimensions array of identical elements being fabricated on the thin layer of silicon wafer. Every element is wired with internal electrodes with no hard wiring needed.
Available Design of Acoustic Microsensors
Two most common designs of MEMS based acoustic transducers are piezoelectric-type and capacitive-type. Both design employed different sensing mechanism and of course working philosophy and materials are also differ. Sound radiating element in piezoelectric type transducer consists of multi layered diaphragm actuated by piezoactive layer.
Piezoelectric usually found in ferroelectric materials with high piezoelectric constant and strong electromechanical coupling factors. PZT are most popular material which sustain superior dielectric constant and thermally stable. Piezoelectric layer in the diaphragm is poled in the thickness direction by applying electrical field. With the present of electrical field across the thickness, strain induces the diaphragm to bend and causing the pressure disturbance with the medium in contact with the transducer. During flexural motion of the diaphragm, charge displacement took place in electroded piezoelectric layer and will be detected by receiving circuitry.
Capacitive type transducer consists of miniaturized parallel plate capacitor. The dimensions of the plate is around 10’s of micrometer with the gap between plates in several 10’s to 100’s of nanometer. Capacitance value for this type of transducer usually in the order of picoFarad.
The back plate position is fixed and the top plate is moving or vibrating under the effect of electrostatic attraction between oppositely charged plate and the opposing restoring force provided by the stiffness of the flexing diaphragm. Basic performance parameter such as bandwidth, dynamic range and electromechanical coupling coefficient was found parallel with the conventional design. The working philosophy is as simple as the change in capacitance value when a top plate vibrating in corresponds to the received acoustic pressure .
There are several limitations exist in capacitive type transducer design. One of those is voltage bias that needs to be apply in achieving desired coupling coefficient usually near exceeding collapse voltage. Using current existing fabrication technology, it is difficult to secure intended safety margin of the bias voltage to avoid collapse in all array element. Piezoelectric type however, does not require any bias voltage.
Capacitive type transducer also need different design for receive and transmit operation. In maximizing the sensitivity, receiving element need to have a thin gap and transmit element need to has large gap to allow large plate deflection. Compare to piezoelectric type transducer, capacitive types was proven to have better sensitivity.
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